There's some confusion in the wake of Apple releasing iOS 4.1 for iPhone and iPod touch to developers today -- namely where is the iOS 4.0.1 bug fix release and where is iOS 4.x for iPad?
iOS 4.1 will be a proper new release, just like iOS 3.1 (iPhone 3.1) was last year, which means developers need to test against it, which means Apple needs to release an early beta for those developers. Last year, iOS 3.1 beta 1 was released on June 30, 2009. So today is a couple weeks earlier than last year's point release beta -- read into that what you will -- but not crazily off schedule.
There will likely be a few more betas, maybe every 2 weeks or so, until September when Steve Jobs takes the stage for the annual special iPod and iTunes music event, announced iOS 4.1 proper, shows off a 4th gen iPod touch and a few new cool features they've kept in their back pocket. Remember, Apple is secretive but they like their cycles. The greatest indicator of what Apple will do this year is what they've done in previous years.
Which brings us to iOS 4.0.1. Again, iOS 3.0.1 (iPhone 3.0.1) was released to the public on July 31, 2009, and there's no reason to think iOS 4.0.1 won't be released late July, early August 2010. Apple's open letter on the antenna issue said a few weeks, that would be a few weeks.
As to iOS 4.x (4.0 or 4.1) for the iPad, that's the wildcard. It didn't exist last year so it has no cycle. If Apple wants to make a big deal about it, they'll announced it at the September show, highlight some features, release a beta that day, and the OS some time later (maybe November). We can only wait and see.